Field of the Invention
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for providing an axial multistage expander.
Description of Related Art
During the past years, with the increase in price of fossil fuels and the enhanced awareness of the public towards energy problems, the interest in developing new technologies or optimizing the existing technologies for reducing energy waste has dramatically increased. Thus, the various energy intensive industries are now looking at ways of reducing their dependency on non-renewable energy sources and also at ways of reducing the energy consumption.
One such way for reducing the energy consumption is minimizing the energy wasted during the production phase. For example, the power industry generates energy by burning coal or natural gas or by using a nuclear reaction for producing heat. After heat from these processes is used for generating the electricity, hot gases are released to the environment. These hot gasses (waste gas) are the carrier of the waste energy. Other examples of large producers of waste energy include blast furnaces, cryogenic devices, oil refineries, chemical plants, etc.
Expanders are used to recover energy from process tail or waste gas. The energy recovered by the expander, which would otherwise be wasted, is used to drive other equipment needed for the process (air compressor) or to produce electricity (generator drive) thereby increasing the efficiency of the plant. Common applications of the expander include: steel mills (blast furnace), air separation plants (cryogenic), oil refineries, chemical plants (nitric acid, ethylene oxide), etc.
An expander 10 is illustrated in FIG. 1 and this figure illustrates not only the size of such a device but also the main components of the expander, i.e., outer casing 12, nose cone 14, stator 16, and diffuser 18. Another component of the expander that is not shown in FIG. 1 is the rotor. FIG. 2 shows an open expander exposing the rotor 20 having plural rotoric airfoils 20a. FIG. 2 also shows the stator 16 having plural airfoils 16a. While the plural statoric airfoils 16a are fixed relative to the casing 12, the plural rotoric airfoils 20a are configured to rotate with rotor 20 relative to casing 12. The rotor 20 is connected to a shaft 22 that facilitates the rotation of the rotor.
While in use, expander 10 is configured to receive a gas along arrows 24. The gas passes over the nose cone 14 of the expander, into the statoric airfoils (blades) 16a and impacts the rotoric airfoils 20a. The rotoric airfoils extract the energy from the gas and convert it into rotational energy through the rotor 20 and shaft 22. This expansion process results in a temperature drop in addition to recovery of the pressure energy.
FIG. 2 shows a single stage expander, i.e., a single rotor part 20 having a single set of airfoils 20a. However, it is possible to have multiple rotor parts having corresponding airfoils, all disposed on a same shaft and all rotating with a same angular speed as shown in FIG. 3. FIG. 3 shows an axial multistage expander 25 having a casing 26, a rotor 27 on which plural rotoric airfoils 28, corresponding to the multiple stages, are fixed so that airfoils 28 rotate together with the rotor 27. Corresponding statoric airfoils 29 are attached to a static part, e.g., the casing 26. Also shown in FIG. 3 is an inlet 30 and an outlet 32 of the expander 25.
However, to achieve the axial multistage expander with a high efficiency in the whole operating range and not to compromise the manufacturability (i.e., to maintain a reasonable height of the airfoils of the first stages), a high rotational speed of the expander rotor is required for the first stages. As the devices that are connected to the expander (e.g., power generator) require in general a lower rotation speed, traditionally, an external gearbox is placed between the expander and the device connected to the expander for matching the different rotational speeds.
In another application, two different axial expanders are used for recovering more hot energy. A first expander is a high speed axial expander (hence higher component stress and costs and requires better grade materials) coupled to a second lower-speed expander with a gearbox unit interposed between the two expanders. This design is illustrated in FIG. 4 in which the first axial expander 34 is coupled to the second axial expander 35 via an external gearbox 36. It is noted that the gearbox 36 is connected, outside expanders 34 and 35, to a shaft 38 of the first expander 32 and a shaft 40 of the second expander 34. This arrangement reduces the overall efficiency of the system and increases its complexity while potentially limiting the operability of the turbomachinery, and also increases the cost of the whole system and the maintenance as well.
Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks.